Flutter/Dart 如何在 QuerySnapshot 的一个字段中有两个 e 两个 isGreaterThanOrEqualTo？

Question

我想从 firestore 的集合中仅 Futerbuilder 中的这些文件具有相同的 data.But 它不起作用。显然是因为我用了两个 isGreaterThanOrEqualTo.

如何在一个字段中使用两个 isGreaterThanOrEqualTo？

Future getData() async {
 var firestore =  Firestore.instance;

QuerySnapshot qn = await firestore.collection(tag).where('ckey', isGreaterThanOrEqualTo: 1).where('dkey', isGreaterThanOrEqualTo: 1).getDocuments();

return qn.documents;
}
...

   void initState() {
    super.initState();
     _data = getPosts();
  }

...

FutureBuilder(
   future: _data
   builder: (context, projectSnap)
        ...
        return ListView.builder(
          itemCount: projectSnap.data.length, 
          itemBuilder: (context, index)

NoSuchMethodError: The getter 'length' was called on null

Answer 1

你有 NoSuchMethodError: The getter 'length' was called on null，因为 FutureBuilder 的快照数据在未来解决之前将为空。

您可以提供 initialData:

FutureBuilder(
   future: _data,
   initialData: [],
   builder: (context, projectSnap) {
        ...
        return ListView.builder(
          itemCount: projectSnap.data.length, 
          itemBuilder: (context, index)

或检查构建器中是否存在数据：

FutureBuilder(
   future: _data,
   builder: (context, projectSnap) {
        if (!projectSnap.hasData) return Container();
        ...
        return ListView.builder(
          itemCount: projectSnap.data.length, 
          itemBuilder: (context, index)

Flutter/Dart 如何在 QuerySnapshot 的一个字段中有两个 e 两个 isGreaterThanOrEqualTo？

Flutter/Dart How can I have two e two isGreaterThanOrEqualTo in one field in a QuerySnapshot?

dart

flutter

flutter-test